Extrusion is a frequently used method for the production, treatment and processing of polymers. Here and in the following, extrusion is understood to refer to the treatment of a medium in a single- or multi-screw extruder.
Extrusion is employed industrially in the production of polymers for the removal of volatile components such as monomers and oligomers, as well as auxiliary agents and solvents from media containing polymers ([1], p. 192 to 212; [1]=Klemens Kohlgrueber, Twin-Screw Extruders, Hanser Publishers, Munich 2007). If required, the polymer can also be chemically modified during extrusion, such as by grafting, modification of functional groups or by modifying the molecular weight, by targeted buildup or reduction of the molecular weight, or the polymer can be converted, e.g. by admixing additives.
The advantages of the extrusion must be weighted against the disadvantage that a particularly high amount of energy is dissipated into the polymer-containing media to be extruded, particularly in the ridge areas of the screw elements typically used as treatment elements in extruders, which may lead to intense local overheating. Such local overheating may damage the product, e.g. by changes in the smell, color or chemical composition, or lead to the development of inhomogeneities in the product such as gel bodies or specks.
Damage patterns of various polymers due to local overheating are referred to, for example, in WO2009/153000 A and described on p. 22, line 7, to p. 24, line 25.
In particular, rubber types such as polybutadiene (BR), natural rubber (NR), polyisoprene (IR), butyl rubber (IIR), chlorine butyl rubber (CIIR), bromobutyl rubber (BIIR), styrene butadiene rubber (SBR), chloroprene rubber (CR), butadiene-acrylonitrile rubber (NBR), partly hydrogenated butadiene-acrylonitrile rubber (HNBR) and ethylene propylene diene copolymers (EPDM) tend to interlink and develop a gel if the temperature is too high, which results in a massive deterioration of the mechanical characteristics of the products produced from these. Where chlorine butyl rubber and bromobutyl rubber, as well as chloroprene rubbers are involved, increased temperatures may lead to the release of hydrogen chloride or hydrogen bromide, which, in turn, catalyzes further disintegration of the polymer.
The reaction speed of any damage to the polymer depends on the temperature. The reaction rate constant k(T) for this may be described using Arrhenius' equation:k(T)=A*exp(−EA/(R*T)).
In this equation, k is the reaction rate constant, T is the absolute temperature in [K], A is the frequency factor, EA is the activation energy in [J/mol] and R is the universal gas constant in [J/(mol*K)].
As well, from an energy-related point of view, methods for the extrusion of media containing polymers should therefore generally be designed in such a way that the average temperature increase is as low as possible, and local temperature peaks, as they occur, for example, in the ridge areas of a screw element with a traditional Erdmenger screw profile in accordance with the current state of technology, are avoided.
It is still advantageous, particularly for the removal of volatile components from media containing polymers, such as solvent residues or water, to attain a high degree of surface renewal via the screw geometry, which facilitates the removal of volatile components.
A number of approaches to addressing the solution of these problems are to be found in the prior art.
A twin-shaft screw machine with single-start treatment and screw elements is known from DE 1 180 718 A. In a sectional view, the outside contour of the screw elements is composed of arcs. The active edge positioned in the direction of rotation has an outside contour composed of three arcs whose centers are either on the external radius or on the longitudinal axis of the screw elements. One disadvantage is that the screw elements allow only a small amount of flexibility for adjusting the shear and/or elongation flow affecting the material to be processed.
WO2009/152968 and WO2011/039016 disclose treatment methods for extruders, such as, in particular, screw elements, which generate a lower degree of energy input into the materials containing polymers during extrusion due to their rounded shape.
A treatment system and a method for degassing bimodal polyolefins are known from EP 1 617 985 A1. Two parallel moving twin-shaft extruders are arranged consecutively in the treatment system, wherein the second extruder, viewed in the direction of flow, has a degassing zone to degas the polyolefins to be processed. The disadvantage of this treatment system is that the degassing performance, i.e. the extent of the degassed proportion of undesired volatile components, is low.
A method and a system for processing strongly degassing materials are known from EP 0861717 A1. The extrusion device has a main extruder and two secondary extruders opening into this at the side, so that the gas current developing in a vaporization zone of the main extruder is separated into at least three partial flows, which are then discharged from the extruders.
EP 1 127 609 A2 discloses a method for removing volatile components from a medium containing polymers using a kneader. Here, the energy is partly introduced via the kneader wall and is used to vaporize the solvent. In addition, energy, as mechanical energy, is introduced by the rotating shaft of the kneader. The introduction of mechanical energy via the kneader is heavily dependent on the viscosity of the product, which reduces flexibility and thus the attractiveness of the method for industrial application markedly.
EP 1 165 302 A1 discloses a device and a method for degassing plastics, comprising a rear degassing zone and multiple degassing zones in the direction of flow, which are operated under a vacuum. The vacuum is required to obtain low residue concentrations of volatile components.
The direct degassing of rubber solutions using a flash tank and one or more extruders is disclosed in “Process Machinery”, Part I and II, March and April 2000; Author: C. G. Hagberg, as well as in WO2010/031823 A and PCT/EP2011/054415.
U.S. Pat. No. 4,055,001 discloses a method for producing polymers such as butyl rubber with a water content of less than 0.1% w/w, using ultrasound sonotrodes during the drying process. The extremely high shear impact due to ultrasound, however, is not favorable for commercial application.
US 2001/056176 A1 discloses a single-step method for concentrating rubber solutions. Here, the rubber solution is heated with steam in order to remove existing solvents in one step by degassing under a vacuum, generating white crumbs. In the process, US 2001/056176 A1 requires a large volumetric steam flow to remove volatile components at low steam pressure and this leads to the undesired inclusion of additional water inside the crumbs. Screw elements for treating polymer melts are known from EP 0 764 076A, which, even at low temperatures, are intended to contribute to the dynamic kneading of such melts during extruder operation due to their asymmetrical geometries.
The abovementioned approaches to solutions, however, cannot be transferred to the extrusion of media containing elastomers or can be improved upon.
A method for degassing media containing polymers, including, in particular, polymer melts, polymer solutions and dispersions, together with degassing devices for performing the abovementioned method, wherein the screw geometry must fulfill certain geometric requirements in order to obtain an improved degassing result, is known from PCT/EP2012/069201. The task underlying the invention was to provide a method for removing volatile components from media containing elastomers, which enables a high degassing capacity combined with a high elastomer throughput while simultaneously resulting in a low residual volatile component content.